Trough collector with concentrator arrangement

ABSTRACT

The invention relates to a trough collector with a primary concentrator, which concentrates solar radiation into a focal line region, with an arrangement for secondary concentration of the solar rays reflected by the primary concentrator, which arrangement concentrates the solar rays further into focal point regions. The arrangement for secondary concentration has a plurality of rows of secondary concentrators which have identical orientation in a row, but have differing orientation from row to row. Further, means are provided in order to keep one of the rows in the operating position and the other rows in a rest position. Thus, a range of the skew angle can be assigned to each of the rows and in the event of the change thereof, another row is brought into operating position.

The present invention relates to a solar collector with a concentratorarrangement according to the preamble of claim 1.

Trough collectors are used inter alia in solar power plants, whereinarrangements for the secondary concentration for such trough collectorshave been increasingly suggested.

Until now it has not been possible to generate solar electricity in anapproximately cost-covering manner by using this technology, owing tothe disadvantages of photovoltaics which have not been overcome. Bycontrast, for some time, solar power plants have already been producingpower on an industrial scale at prices which, compared to photovoltaicmethods, are close to the commercial prices now usual for power producedin the conventional manner.

In solar thermal power plants, the radiation of the sun is reflectedusing the concentrator through collectors and focused in a targetedmanner on a location in which high temperatures (or high light density)arise as a result. The concentrated heat can be conducted away and usedto operate thermal engines such as turbines which in turn drive thegenerators which generate electricity.

Three basic forms of solar thermal power plant are currently in use:dish/Stirling systems, solar tower plant systems and parabolic troughsystems.

The dish/Sterling systems as small units in the range of up to 50 kW permodule have generally not caught on.

Solar tower plant systems have a central absorber which is mounted in anelevated manner (on the “tower”) for the sunlight which is reflected toit by means of hundreds to thousands of individual mirrors, whereby theradiation energy of the sun is concentrated in the absorber by means ofthe many mirrors or concentrators and thus temperatures of up to 1300°C. should be reached, which is favourable for the efficiency of thedownstream thermal engines (generally a steam or fluid turbine powerplant for electricity generation). The “Solar two” system in Californiahas an output of several MW. The PS20 system in California has an outputof 20 MW.

Solar tower power plants have hitherto likewise not become relativelywidespread (in spite of the high temperatures which can advantageouslybe reached).

Parabolic trough plants are however widespread and have large numbers ofcollectors which have long concentrators with small transversedimensions and thus do not have a focal point but a focal line. Theselinear concentrators currently have a length of 20 m to 150 m. Anabsorber pipe runs in the focal line for the concentrated heat (up toalmost 500° C.), which transports the heat to the power plant. Thermaloil, molten salts or superheated steam for example are possibilities forthe transport medium.

The 9 SEGS parabolic trough plants in Southern California togetherproduce an output of approx. 350 MW. The power plant “Nevada Solar One”,connected to the mains in 2007, has trough collectors with 182,400curved mirrors, which are arranged on an area of 140 hectares, andproduces 65 MW. Andasol 3 in Spain has been under construction sinceSeptember 2009 and should enter into operation in 2011, so that theplants Andasol 1 to 3 will have a maximum output of 50 MW.

For the plant as a whole (Andasol 1 to 3), a peak efficiency of approx.20% and also an annual average efficiency of about 15% is expected.

As indicated above, an essential parameter for the efficiency of a solartower power plant is the temperature of the transport medium heated bythe collectors, by means of which transport medium the heat obtained istransported away from the collector and is used for conversion intopower for example: at relatively high temperature, a higher efficiencycan be achieved during the conversion. The temperature that can berealised in the transport medium depends in turn on the concentration ofthe solar radiation reflected by the concentrator. A concentration of 50means that in the focal range of the concentrator, an energy density perm² which corresponds to 50-times the energy irradiated from the sun to am² of the earth's surface is achieved.

The theoretically maximum possible concentration depends on theEarth-Sun geometry, that is to say on the opening angle of the solardisc observed from the Earth. It follows from this opening angle of0.27° that the theoretically maximum possible concentration factor liesat 213 for trough collectors.

Even with mirrors which are very complex in terms of production andtherefore (too) expensive for industrial deployment and are wellapproximated to a parabola in cross section and therefore produce afocal line range with smallest diameter, it is not possible today toeven reach this maximum concentration of 213 only approximately. Areliably achievable concentration of approx. 50 to 60 is howeverrealistic and already allows the above-mentioned temperatures ofapproximately 500° C. in the absorber pipe of a parabolic trough powerplant.

To increase the achievable temperature, secondary concentrators have nowbeen suggested, which once again concentrate the solar radiationreflected by the primary (trough) concentrator in the longitudinaldirection of the primary concentrator, so that the solar radiation isultimately concentrated into a number of focal points, thus theconcentration of the sunlight and the thus created temperature arehigher and more than 600° C. should be achievable.

The same is true if the radiation should be concentrated ontophotovoltaic cells, which however, as mentioned above, has hitherto notbeen realised on an industrial scale.

The construction of secondary concentrators is demanding however, as theso-called skew angle, i.e. the angle at which the sunlight falls ontothe primary concentrator of a trough collector, changes seasonally andover the day, wherein the focal point regions created by the secondaryconcentrators can then shift for example, which is problematic inparticular in the case of the use of an absorber pipe with thermalopenings.

Secondary concentrators constructed as compound parabolic concentrators(CPCs) appear particularly suitable for a concentration in thelongitudinal direction, but have the disadvantage that the achievableconcentration is dependent on the angle of acceptance (θ_(in)) of thesecondary concentrator (radiation which enters the secondaryconcentrator outside of the angle of acceptance does not reach the focalpoint region created thereby): the greater θ_(in) is, the smaller is thefurther concentration achievable by means of the CPC secondaryconcentrator.

It was suggested in US 2010/037953 to arrange the secondaryconcentrators, which are constructed as Fresnel lenses or parabolicreflectors, pivotably with respect to the primary concentrator, so thatthese can constantly be made to track the current skew angle.

The solution shown for the construction of the pivotable secondaryconcentrators has the disadvantage however that these can only bepivoted over a small part of the necessary pivoting range, withoutstriking one another and thus blocking a further pivoting movement. Itis conceivable to equip the arrangement shown with secondaryconcentrators which are spaced apart in the vertical position, whichalthough it would allow the necessary pivotability, has the consequencethat not all of the solar radiation reflected by the primaryconcentrator can be secondarily concentrated in the vertical positionrequired during operation, which reduces the efficiency of the solarcollector.

Accordingly, it is the object of the present invention to provide atrough collector with an improved arrangement for the secondaryconcentration of reflected solar rays.

This object is achieved by means of a trough collector with the featuresof claim 1.

The fact that, for incident radiation, variously orientated componentscan be positioned alternately in the path of the radiation reflected bythe primary concentrator enables the use of secondary concentratorsorientated in accordance with the current skew angle, without the samehaving to be of pivotable construction. Secondary concentrators ofdifferent orientation not currently required can be parked in a restposition outside of the path of the reflected radiation.

Accordingly, the constructive outlay is dispensed with for anarrangement of pivotable secondary concentrators both for the completecapture of all reflected rays and for the orientation which is always tobe observed with high precision with the passage of time, but isconstantly changing. This is of particular importance in ahigh-temperature environment, as is naturally unavoidable in the case ofsecondary concentrators which should generate temperatures above 600°.Likewise, or to a greater extent if photovoltaic cells are used, whichby their very nature must be arranged directly at the exit of thesecondary concentrators and therefore must be cooled, which createsadditional design problems.

The invention is explained in more detail on the basis of the figures.

In the figures:

FIG. 1 a schematically shows a trough collector of known design with anarrangement of secondary concentrators,

FIG. 1 b schematically shows the daily path of the sun and the skewangle occurring,

FIG. 1 c schematically shows the skew angle in the collector,

FIG. 1 d shows a graph with the change of the skew angle over a year ina North/South orientation of a trough collector with the assumedlocation of Dubai,

FIG. 2 shows a preferred embodiment of a secondary concentrator,

FIG. 3 schematically shows a first preferred embodiment of the troughcollector according to the present invention,

FIG. 4 shows a view from above onto the embodiment of FIG. 3,

FIG. 5 shows a view onto a section of the adjacently arranged rows ofsecondary concentrators according to the view of FIG. 4,

FIG. 6 shows a further embodiment according to the present invention,and

FIGS. 7 a to 7 c show a view from the side of a concentrating elementmodified according to the invention for various acceptance ranges.

FIG. 1 a shows a trough collector 1 according to the prior art with aprimary concentrator 2 which rests in a frame which is not illustratedin any more detail so as not to overload the figure, is of pivotableconstruction and thus can be made to track the daily course of the sun.

The double arrow 3 shows the longitudinal direction, the double arrow 4shows the transverse direction of the trough collector 1 and the doublearrow 5 shows the pivoting directions of the collector 1.

Further illustrated is an arrangement 8 of secondary concentrators 9here constructed as Fresnel lenses, and also a solar ray 10 which fallsonto the primary concentrator 2, is reflected by means of the same as aray 11 towards a focal line region of the primary concentrator 2 andafter the passage through a secondary concentrator 9 is refracted in thelongitudinal direction 3, so that it is finally directed as ray 12 ontoa focal line region 13.

In other words, the incident solar rays are initially concentrated inthe transverse direction 4, then in the longitudinal direction 3,wherein the thus arising focal point regions 13 are located on anabsorber pipe 14 which absorbs the heat and dissipates the same via aheat-transporting medium.

The primary concentrator 2, here illustrated as a rigid mirror, can alsobe constructed as a flexible film clamped in a pressure cell, as isillustrated for example in WO 2010/037243. Instead of the absorber pipe14 shown here, photovoltaic cells may also be provided for producingpower at the location of the focal point regions 13.

As mentioned at the beginning, it is suggested in US 2010/037953 toconstruct the secondary concentrators 9 in a pivotable manner with apivot axis running in the transverse direction 4, in order to constantlymake the same track the skew angle (see FIG. 1 b).

FIG. 1 b shows the daily path of the sun in relation to a collector 1.Illustrated is the collector 1 orientated in the North/South directionwith the horizon symbolised by the dashed line 20, as may be visiblefrom the collector 1. Further illustrated is the path 21 of the sun on asummer's day which begins in the East at point 22 and ends in the Westat point 23. Likewise, the path 25 of the sun on a winter's daybeginning in the East at point 26 and ending in the West at point 27 canbe seen.

By pivoting in accordance with the double arrow 5, the collector 1 iscontinuously orientated towards the sun over the day, i.e. in themorning, it is tilted to the left with reference to the FIG. 1 b,orientated horizontally at midday and tilted to the right in theevening.

In spite of this orientation, it is the case that in the summer the sun(here seen with reference to the drawing plane) rises in front of thecollector 1 (that is to say North thereof), is behind the same (that isto say to the South) at midday and goes down in front of the same (thatis to say in turn to the North) in the evening. In the winter, the sunis constantly behind the collector 1, that is to say to the Souththereof.

This is illustrated by means of the normal 28 of the collector 1 whichlies perpendicularly on the surface line 29 of the concentrator 2 whichis marked dashed and runs in the longitudinal direction 3: for exampleit encloses a first angle S with the solar ray 30 (sun in winter) and itencloses a second angle S with the solar ray 31 (sun in summer). Theangle S is known to the person skilled in the art as the skew angle anddesignates the incidence of the solar rats in the longitudinal direction3 obliquely onto the concentrator of the collector, when the same isorientated towards the sun.

If a solar ray 31 falls from obliquely in front onto the collector 1,the skew angle is negative, if a solar ray 30 falls from obliquelybehind onto the collector 1, the skew angle is positive. If a solar raycoincides with the normal 29 at midday, the skew angle is 0. This isshown in summary in FIGS. 1 c and 1 d: If the collector 1 is orientatedtowards the sun, the solar rays fall below the skew angle S onto theconcentrator 2 in such a manner that they lie with the normal 29 in aplane E, wherein the reflected rays, which are not illustrated so as notto overload the figure, are concentrated into the focal line region ofthe concentrator 2.

FIG. 1 d shows a graph by way of example with the region of the skewangle based on the location of Dubai depending on the season: the seasont is plotted on the horizontal axis, the value of the skew angle indegrees is plotted on the vertical axis.

The graph of FIG. 1 d is based on a North/South orientation of thecollector 1, wherein the pivoting range of −70 to +70° is sufficient (0°corresponds to the horizontal orientation at midday).

From the relationships illustrated in FIG. 1 b, it is possible to readoff that the skew angle S in summer, for example on 25 June, in themorning (point 22 in FIG. 1 b) at approx. −17°, at midday at approx. +4°and in the evening again at −17°. Over the day, the skew angle S changesby approx. 21°.

In winter, for example on 5 January, the skew angle S changes betweenapprox. +32° and approx. +48°, changes therefore by approx. 26°.

FIG. 2 shows a secondary concentrator 40, as can be used in a collector1 (FIG. 1 a) in the place of the secondary concentrators 9 constructedas Fresnel lenses shown schematically there. The secondary concentrator40 has a front wall 41 and a rear wall 42, which are constructed ascompound parabolic concentrators (CPCs). CPCs are fundamentally known tothe person skilled in the art. The CPC is used for the secondaryconcentration of the solar rays 11, 11′ reflected by the primaryconcentrator 2 (FIG. 1 a), i.e. it concentrates the same in thelongitudinal direction 3.

Further illustrated are a right side wall 43 and a left side wall 44,which are constructed as trumpet concentrators and allow solar rays 11concentrated by the primary concentrator in the transverse direction 4to additionally concentrate once more in the transverse direction 4. Atrumpet concentrator is fundamentally known to a person skilled in theart.

As a result, highly concentrated rays 45 emerge from the upper opening46 of the secondary concentrator 40 and according to the invention in afocal point region 47 hit an absorber pipe 14 (FIG. 1 a) or photovoltaiccells, which are omitted so as not to overload the figure.

The lower opening 48 of the secondary concentrator 40 has an acceptancerange which is determined by the properties of the CPCs and also thetrumpet concentrator, with the consequence that only rays 11 incidentbelow the acceptance angle are concentrated into the focal point region47, which is not the case for the ray 11′ lying outside the acceptanceangle.

FIG. 3 shows a preferred embodiment of the present invention.Illustrated is a collector 50 with a flexible concentrator membrane 52arranged in a pressure cell 51, which primarily concentrates theincident solar rays 53, 53′ and therefore reflects the same as rays 54,54′ onto an arrangement for secondary concentration 55. The pressurecell 51 is clamped in a frame 59 of the collector 50.

In a framework 56 below the absorber pipe 57, a carriage 58 is arranged,which is arranged such that it can be displaced back and forth in thetransverse direction 4 below the absorber pipe 57 and carries secondaryconcentrators 40 (FIG. 2), here in a plurality of mutually adjacent rows60, 61 and 62, so that the secondary concentrators 40 are grouped in therows 60 to 62. In each row or group 60 to 62, the associated secondaryconcentrators 40 then lie one behind the other and are thus arrangedalong the length of the primary concentrator.

So as not to overload the figure, a conventional drive of the carriage58, which can readily be conceived by the person skilled in the art inaccordance with the requirements on site, is omitted.

By means of suitable movement of the carriage 58, in each case one ofthe rows 60 to 62 is located below the absorber pipe 57, i.e. in theoperating position in the path of the reflected radiation and the twoother rows are located in the rest position i.e. outside of the path ofthe reflected radiation.

The secondary concentrators of each row (or group) 60 to 62 areorientated differently compared to those of another group, that is tosay have a differently orientated acceptance range and are thereforesuitable to secondarily concentrate radiation which corresponds to anassociated predetermined range of the skew angle S, i.e. in apredetermined skew range:

Starting from the range of the skew angle at the actual location andaccording to the graph of FIG. 1 d, the person skilled in the art cantherefore determine the complete skew range (which in the example ofFIG. 1 d ranges from −18° to)+49°, divide the same into a suitablenumber of predetermined skew ranges (in the example according to thearrangement of FIG. 3, three thereof) and assign a row (or group) 60 to62 of secondary concentrators 40 to each of these thus predeterminedskew ranges, which secondary concentrators are orientated towards theirskew range and which are then brought into operating position in thecorresponding season by means of the displacement of the carriage 48.

FIG. 4 shows a view from above onto the collector 50 according to FIG.3, wherein the absorber pipe 57 is omitted so as not to overload thefigure. The position thereof is illustrated by means of the line 70.

According to the embodiment illustrated, three rows or groups 60 to 62of secondary concentrators 40 are arranged in the carriage 58. Asmentioned above, the secondary concentrators 40 are orientated in arespective row towards an associated skew range.

A section of the rows 60 to 62 of secondary concentrators, which isillustrated in more detail in FIG. 5, is designated by means of thedashed line 71.

As a result, the trough collector according to the invention has anarrangement 65 for secondary concentration of the solar rays 54, 54′reflected by the primary concentrator 42, which arrangement concentratesthe solar rays further into focal point regions 46 (FIG. 2), wherein thearrangement 65 for secondary concentration of the reflected radiationhas a number of variously orientated concentrating components hereconstructed as secondary concentrators 40 and further has means in orderto bring the concentrating components alternately into an operatingposition in the path of the reflected radiation or into a rest positionoutside of the path of the reflected radiation. These means areconstructed in the embodiment according to FIG. 3 as framework 56,carriage 58 and drive for the carriage 58.

FIG. 5 shows the view onto the section according to the dashed line 71made up of the rows 60 to 62 of secondary concentrators 40. So as not tooverload the figure, the carriage 58 and all further elements of thecollector 50 are omitted, only the position of the absorber pipe 57 isshown by the line 71. In the figure, the different orientation of thesecondary concentrators of each of the rows 60 to 62 can be seenclearly.

As mentioned above, the present invention is not limited to theembodiment of a secondary concentrator illustrated in FIG. 2, anyelement, by means of which the radiation reflected by the primaryconcentrator is concentrated longitudinally into a focal point region,conforms with the invention. Further, the means for the displacement ofthe secondarily concentrating elements may be constructed differently,so for example in the place of a carriage 58 travelling over theframework 56, it is conceivable to arrange the secondarily concentratingelements on rotating rings placed around the absorber pipe. Likewise,photovoltaic cells may be arranged in the focal point regions formed bythe secondarily concentrating elements.

The parabolic-trough shape of the primary concentrator means that thereflected rays do not fall parallel into the secondarily concentratingelement as seen in the longitudinal direction, but rather at an angle ofa few degrees, the value of which changes with the size of the skewangle. Accordingly, in a preferred embodiment, the acceptance ranges ofthe secondarily concentrating elements of the various rows or groups areconstructed in an overlapping manner, so that when changing from one rowto another row, the currently prevailing solar radiation can beconcentrated completely into the focal point regions by both rows.

FIG. 6 shows a further embodiment of a collector 80 according to thepresent invention, in which the arrangement 65 for secondaryconcentration is modified. A single row 83 of secondarily concentratingelements, here of secondary concentrators 40 is fixedly arranged in theframework 56 by means of holding arms 84 between the absorber pipe 57and the carriage 58. Arranged in the carriage 58 are rows 85 and 86 ofattachment elements 87 and 89, which, depending on the position of thecarriage 85 lie in an operating position (i.e. in the path of theradiation 44, 44′) and together with the secondary concentrators 40 forma modified element for secondary concentrators. The effect of theattachment element is such that the acceptance range of the secondaryconcentrators 40 changes so that, in turn, three different rows ofsecondarily concentrating elements exist, which in each case areassigned to a skew range and have the corresponding acceptance range.

Secondary concentrators 40 are shown schematically in the FIGS. 7 a to 7c, wherein FIG. 7 a shows a secondary concentrator 40 without attachmentelement, the acceptance range of which corresponds to the dashed line86. Illustrated in FIG. 7 b is an attachment element 87 whichconstitutes an asymmetric continuation of the front wall opposite therear wall 42 of the secondary concentrator 40 and thus changes thedirection of the acceptance range thereof in accordance with the dashedline 88. Likewise in FIG. 7 c, where the direction of the acceptancerange of the secondary concentrator 40 illustrated there is changed evenmore strongly by means of the larger attachment element 89, as isillustrated by the dashed line 90. For example and in accordance withthe graph according to FIG. 1 c, the row of secondary concentrators 40according to FIG. 7 a can be laid out for a skew range of −18° to +10°,the row of secondary concentrators with attachment elements 87 accordingto FIG. 7 b can be laid out for a skew range of 5° to +33°, and the rowof secondary concentrators with attachment elements 89 according to FIG.7 c can be laid out for a skew range of +30° to +48°.

The person skilled in the art can determine the skew ranges depending onthe actual conditions prevailing on site. Likewise, the person skilledin the art can determine the number of rows of secondarily concentratingelements; although the number of three rows shown in the presentexemplary embodiments is seen as advantageous, only two or more thanthree, for example four to six rows, are conceivable.

1. A trough collector comprising: a primary concentrator, whichconcentrates solar radiation into a focal line region, with anarrangement for secondary concentration of the solar rays reflected bythe primary concentrator 2, which arrangement concentrates the solarrays further into focal point regions; wherein the arrangement forsecondary concentration of the reflected radiation has a number ofcomponents variously orientated for incident radiation and means inorder to bring the variously orientated components alternately into anoperating position in the path of the reflected radiation or into a restposition placed outside of the path of the reflected radiation.
 2. Thetrough collector according to claim 1, wherein the variously orientatedcomponents in groups have the same acceptance range fixedly orientatedtowards a predetermined skew range, a plurality of groups are providedfor in each case different skew ranges and means are constructed toposition in each case one of the groups operationally in the path of thereflected radiation.
 3. The trough collector according to claim 1,wherein the acceptance range of at least one group in the transversedirection is constructed differently to that of another group.
 4. Thetrough collector according to claim 1, wherein the concentratingcomponents are constructed as secondary concentrators with identicalorientation within the group, but differing orientation from group togroup, wherein the orientation of a group corresponds to a predeterminedskew range and the secondary concentrators of a group are arranged onebehind the other along the length of the primary concentrator.
 5. Thetrough collector according to claim 1, wherein the concentratingcomponents have a single group of fixedly orientated secondaryconcentrators and various groups of attachment elements for thesecondary concentrators, which in connection with the secondaryconcentrators in each case effect the orientation of the acceptancerange thereof for various skew ranges, and wherein the means areconstructed to position in each case one group of attachment elementsoperationally in the path of the radiation upstream of the secondaryconcentrators.
 6. The trough collector according to claim 4, wherein thesecondary concentrators are constructed as compound parabolicconcentrators.
 7. The trough collector according to claim 1, wherein thearrangement for secondary concentration is constructed for theconcentration of the reflected radiation in the transverse directionalso and preferably has a trumpet concentrator for each focal pointregion.
 8. The trough collector according to claim 1, wherein thearrangement for secondary concentration is constructed to concentratesunlight, which is incident at a skew angle of −20° to +50° onto theprimary concentrator, into focal point regions.
 9. The trough collectoraccording to claim 2, wherein the acceptance ranges for the variouspredetermined skew ranges overlap.
 10. The trough collector according toclaim 1, wherein an absorber pipe is provided with thermal openings andthe focal point regions lie at the location of the thermal openings. 11.The trough collector according to claim 1, wherein photovoltaic cellsare arranged at the location of the focal point regions.
 12. The troughcollector according to claim 1, wherein the primary concentrator isdivided into a plurality of longitudinal regions and an arrangement forthe secondary concentration of the sunlight reflected by thelongitudinal region is assigned to each longitudinal region.